Second order differential microphone



Nov. 7, 1950 A. M. WIGGINS 2,529,467

SECOND ORDER DIFFERENTIAL MICROPHONE Filed Aug. 4, 1948 2 Sheets-Sheet 1 Cl 4""B P2 HvCb c2 gmse P g 2 j P3 1 15.5 gig INVEN TOR. 46 A 1. FHA M Mam/v.5.

Nov. 7, 1950 A. M. WIGGINS SECOND ORDER DIFFERENTIAL MICROPHONE 2 Sheets-Sheet 2 Filed Aug. 4, 1948 INVENTOR. 4 LPHA M, W/GG/(VS Patented Nov. 7, 1950 SECOND ORDER DIFFERENTIAL MICROPHON E Alpha M. Wiggins, Clay Township, St. Joseph County, Ind., assignor to Electro Voice,- Inc., Buchanan, Mich., a corporation of Indiana Application August 4, 1948, Serial No. 42,369

This invention relates to improvements in differential microphones.

Diiierential microphones as here considered are of the generaltype illustrated in and disclosed in Patent No. 2,350,010, granted May 30, 1944, to F. C. Beekley. A microphone of this type may be defined as a pressure gradient microphone which has two or more sound entrances spaced by an acoustic distance which is small compared to the wave lengths of sound which may pass through said entrances to impinge upon sound responsive generating means incorporated in the microphone, whereby the resultant of the forces acting on the generating means in response to random sounds of distant origin is attenuated, while the resultant of the forces acting on the generating means in response to sounds of close origin is preponderantly that resulting from the sound pressure at the entrances nearest to the sound source. In other words, a difierential microphone is one having a proximity effect rendering the microphone highly sensitive to sounds of close origin directed preponderantly at one sound 'entrance and much less sensitive to random sounds of distant origin, so that sounds of close origin are transmitted intelligibly, and random sounds of distant origin are substantially inefiective upon the generating means or have such diminished effect thereon as not to detract materially from the intelligibility of the translation of the sounds of close origin by the microphone.

This invention relates particularly to a second order difierential microphone which has a very high discrimination against random sounds of distant origin and which is responsive only to the difference between two differential resultant forces of pressure gradients. One form of such a second order differential'microphone is illustrated in my co-pending application, Ser. No. 775,869,

filed September 24, 1947.

The primary object of this invention is to provide a novel, simple and inexpensive microphone having a discrimination of the second order which requires only one sound responsive membrane or 12 Claims. (Cl. 179-135) sitely with respect to the mouth of a passage leading to the opposite surface of the membrane.

Other objects will be apparent from the following specification.

' In the drawing:

Fig. 1 is a face view of one embodiment of the invention. v

Fig. 2 is a transverse sectional view taken on line 2-4 of Fig. 1.

Fig. 3 is a diagrammatic View similar to Fig. 2 illustrating the forces and the electrical conversion values encounteredin the operation of the device. I

Fig. 4 is the equivalent electrical circuit of the device. V

Fig. 5 is a diagrammatic View illustrating pictorially the application of the forces and the angular relationship thereof in the operation of the device. V 7

Fig. 6. is a diagrammatic illustration of the forces acting upon the device. 7

s Fig. Us a face view of another embodiment of the invention with parts thereof shown in section, taken online 1 'l of Fig.8. 7

Fig. 8 is a transverse sectional view taken on Fig. 9 is aschematic'view of the embodiment illustratedin Fig 7, illustrating the application of the forces tothe device.

Fig. 10 is a vector diagram of the pressures when applied as illustrated in Fig. 9.

Fig. 11 is a diagrammatic view of a modification of the embodiments of the invention illustrated in Figs. '7 and 8. v

Fig. 12 is a diagrammatic view of another embodiment of the invention illustrated in Figs. 7 and 8. r

Fig. 13 is a sectional view similar to Figs. 2 and 8,1illustrating another embodiment of the invention.

Referring to the drawings,'and particularly to Figs. 1 to fithereof, the numeral l0 designates the housing of a microphone which is of substantially cup-shape and whose open end is adapted to be spanned by a, closure plate l2. The closure plate l2 mountsan annular flange l4 and a diaphragm I 6 spans the flange l4 and is fixedly secured thereto as by the retainer ring I8. The translating medium or generating system is preferably 0f the carbon granule type, and, for this purpose a suitable annular retainer 20, which may be formed ofglass wool or other readily compressible material having certain properties of resilience, is positioned between the diaphragm phragm mounting flange l4 so thatthey may.

pass into impingement with the inner or bottom surface of the diaphragm Is as illustrated. The opening 30 is formed within the outline of the annular flange l4 and provides access for sound to act upon the upper or outer'face of the diaphragm [6. As here illustrated, the dia-- phragm I6 is positioned substantially centrally between the top and bottom walls of the casing, and the openings 28, 30 and 32 are arranged in line; as best seen in, Fig. :1. and 32: are locatedonopposite sides: ofhe p n: ing. 30: and are. spaced" equally therefrom. While carbon button generating system. has been'illustrated herein for purposes of convenience; any other: generating system; such as piezo electric element, a condenser, and ribbon or a. moving coil, may beutilized; if so desired. While the opening 30 has been illustrated as positioned concentrically with'respect to the, diaphragm, that. centered condition. is not essential. The alignment of the openings 28, ill and, 32 for. max, imum second order differential effect is QSSEIIF- tial; however. Departures from, such alignment produce the same action, which will be discussed. hereafter with respect to. the. construction illustrated in. Fig. 13. V

The-microphone illustratedin 1, and. 2 responds to the gradient of the gradientQfsQUnd;

p essure even. though itusesonly the singledlaphragm H5. The equivalentelectrical; circuit of. this. microphone is. shown in. Fi 4... n h conversion. units. occur. and are. deriv d. from e. elements as indicated inE g. .3 Thus,.ref'erring to. Fig... 3athe. openings M I... M2- and. M3 being; in line, are provided to have approggimately'. the same acoustic impedance. .Soundpressurefrom direction A originating. at; a nrgen tan e ay, from the device will arrive atgthe three openings in phase and at "equal amplitudefso that there will be no movement of the diaphragm for sound from this direction. Sound from direction B originating at a large distance away from the microphone will arrive at the openings at equal amplitudes but in different phase relation'. Fig. is a vector diagram of the pressures at the three openings. Thevector diagram of: Fig. 5 applies for any angle of incidence of sound from. direction B" which is coincident with ithe axis of the microphone, ,oryystated', differently,

with which the diaphragm and the planesiof; the mouths. of thethree openings are parallel. 'Referrin-gto. Fig. 5., K equals 21r/x where )e is the wave length. Further referring to the indicia upon Fig; 5, D- is" the'distancebetween thetwo. outside openings" M2 and-M3. or 28-. and 3218.- ferring to-Fig;2. V Fig. 6.. is a diagram of the forces acting; on the diaphragm due tothe sound pressure pectorially illustrated. in Fig. 5.. The, force on the front of thediaphragm isacting. in the opposite direction from the force onthe backof. the. diaphragm, and the. magnitude of thesum. of F2. and F3. is equal to, El. A further. condition, illustrated in. this, dia ram showsthe value 111192;}:23) is lessjthan the force Fl.

T e penings. 2%

The diaphragm responds to the following rule (P2P1) minus (Pl-P3).

Since P2Pl or PIP3 is the pressure gradient, the microphone responds to the difference in the pressure gradient which is the gradient of the pressure gradient.

By referring; to Fig. 5; the. change in pressure which is available for actuating the diaphragm may be written A(. Ap). equals. P (cos cos 0])+ in s 2 KD K D cos T cos 0 minus J 8111 cos 0 Since for small angles cos 0] minus 2 plus KD K D cos 0 cos cos 0 equals 1 minus 2.. 2. A( A equals P cos 0. l

force has a polar curve proportional to 005 0.

The random efficiency may be calculated by the following equation:

Efficiency equals A,.

Eor: close sounds; the pressure available fora-Qtuating the. diaphragm; is. predominantly the.

pressure: at onesound opening;

Assuming that the; pressure; for close talkingis predominantly the. sound pressure; at the, center opening in the" embodiment illustrated, and also; assuming that this sound pressure is the same;-as. that due tov a distant source, the discrimination against. random. sound originating at; a: distance may be written 7 Dis..eq.ua1s: KiD. The discrimination f-o-r eitherv of the. two: open ingsusedfor close talking purposes is:

Dis. equals Ke z As shown from Equation 1, the microphone has" ma be; desirable inv microphones, intended. for

some uses for applications.

Another embodiment of, theinyention. is illustrated in Figs. 7 and. 8. wherein, the microphone. has a housing 40 of. substantially cupeshape closed by a cover plate 42,.and the sidewalls 4.4. of the casing. 40. have a. plurality of. openings formed. therein, constituting the. openingsv 4.6 which, are arranged; diametrically opposite. each other and. the, openings 48. which. are arranged. diametrically oppositely, In. this embodiment the cover 42 has an. annular flange, 50. depend. therefrom which. is spanned.- by the dia-. phrag m 52 securedv in. place by a. retainer ring 54., The annular flange '50. is of. smaller diameter than, the, housing wall ,.4.4..and-is. spaced inwardly therefrom but is provided at points in register with the openings 48 with enlargements 56 which have passages 58 therein communicating with the passages 48 in the cup-shaped housing. The openings 58 lead to the cavity 60 at the upper side of the diaphragm 52 as viewed in Fig. 8. The openings 46 are open for the passage of sound for impingement upon the bottom side of the diaphragm 52 as viewed in Fig. 8. The microphone has a generating element, here shown as a group of carbon granules 62, encased within a retainer ring 54 spanning the space between the diaphragm 52 and the bottom wall of the casing 40, and the bottom wall of the casing 40 has a button 65 therein engaged by said carbon granules 62.

The response characteristics of this microphone are those of a second order differential similar to the conjoint use of two first order differential microphones connected differentially and placed side by side with their diaphragms located in a common plane. One form of such a microphone is illustrated in my co-pending application, Ser. No. 775,869. One of the important characteristics of this microphone is that the four sound openings 46 and 48 are located in the same plane and it will be observed from the construction that two of these openings, namely the openings 46, lead to the back of the diaphragm,

whereas the openings 48 lead through the passage 58 to the front of the diaphragm. The openings 46 and 48 lead to opposite faces, are alternately arranged around the circumference of the microphone, and the spacing of one opening 46 from an adjacent opening 48 should be the same as the spacing between the other opening 46 and the other opening 48. The acoustic result or action of any one pair of openings comprising one opening 46 and one opening 48 is the action of a first order differential microphone, and that of the other pair is the equivalent of the action of another first order differential microphone connected in opposition to' th one first named.

Fig. 9 is a diagram illustrating the manner in which the pressure forces acting upon the microphone at the various openings are applied. Fig. 10 is a vector diagram of the pressures acting on the diaphragm of the microphone. From Fig. 10 it will be observed that the angle of incidence of sound assumed entails a condition where P2 plus P4 is greater than PI plus P3. Also in Fig. 9, D is the distance between opposite entrances to the same side of the diaphragm of the microphone. Pl, P2, P3 and P4 are the pressures at the various sound entrances.

The change in the pressure gradient available for actuating the diaphragm isthe vector sum of all the pressures acting on the diaphragm. This may be written 1 g A equals 2P(cos cos 9] minus cos sin 6]) For small angles cos cos equals 1- The Equation 2 shows that the force available for actuating the diaphragm'is proportional to the square of the frequency and is given by sin 0cos 0.

'The random efliciency is given by the follow 4 2 2 equals JL I (sin 6 7r[10s 0) sin 0d0 f sine d6 0 Elf. equals .182

ingjformula 7 Dis equals {.182 K 1) The holes oropenings need not all be equally spaced around the diaphragm or equispaced around the circumference of the casing, as shown in Figs. 7 and 9. Instead, they may be arranged as illustrated in the diagram in Fig. 11. Such a configuration will give a different polar pattern which may be calculated in the same manner as calculated above.

Another embodiment of this invention is shown in Fig. 12. This embodiment i analogous to a three-phase network, and its polar response may be calculated as in the case of Fig, 9. Other embodiments of the invention are possible by increasing the number of openings above six, as shown in Fig. 12.

In all of these embodiments the openings must be symmetrical, that is, the adjacent pairs leading to opposite sides of the diaphragm must be spaced the same distance apart and must have their mouths located in the same plane to realize the full second order discrimination against distant sounds. If the mouths of the openings are not all in the same plane, there will be a first order effect with its accompanying decrease in discrimination. Such a condition is illustrated in Fig. 13 where a casing 10 has a diaphragm 12 located squarely therein to divide the interior of the chamber into two substantially equal compartments. Access to one of these compartments is provided by two or more openings 14, and access to the other compartmentis provided by an equal number of openings 16. The adjacent pairs ofopenings 14, 16 are spaced circumferentially the same distance as explained above andjare distance 0 R as shown in Fig. 13. 'If the distance 0 R between the openings of the two sets is only slightly out of the same plane, there will be a slight first order effect and the microphone will still be within the spirit of the invention. Thus, while for practical considerations best results are obtained when'the-mouths of the openings are in the same plane, conditions may exist which prevent this ideal arrangement. I Departures from the coplanar arrangement are practical if the total distance longitudinally and circumfer-' entially between the adjacent openings of opposite sets and leading to opposite sides of the diaphragm is greater than twice the distance 0 R by which the mouths of said openings are disaligned from a common plane, that is, twice the spacing of the plane of said openings in a disame first order eifect, assuming that the rethe polar curve the discrimination, even though not as great as e cend Qrder if e a ff ct i l be con iderably more than it would be possible to obtain with a first order differential microphone of equal spacing of sound entrances.

In all of the embodiments illustrated in this application, the generating system has been shown as a carbon granule type for convenience. It is to be distinctly understood, however, that the invention is not limited to microphones utilizing carbon granule generatin systems, and any other type of generating system may be used, such as 'a dynamic system including the voice coil and ribbon types, a magnetic system, a piezo electric system or a condenser system. Also it will be understood that the structural chracteristics illustrated here are illustrative and are not intended to be limiting and that structural changes may be made in the microphone from those herein shown within the scope of the appended claims without departin from the spirit ofthe invention.

I'claim:

1'. A microphone comprising a housing, a single diaphragm mounted in said housing and dividing said housing into two chambers, a generating element' actuable by said diaphragm, a plurality of openings in said housing each having substantially'the same acoustic impedance and substantially the same spacing from said diaphragm, at least one of said chambers being in communication with two of said openings, and at least one opening communicating with the'other chamber, the mouths of all of said openings being located in a common plane and the mouths of the first named openings being equally spaced from the mouth of the last named opening whereby said diaphragm is responsive only to the difference of the difference in the sound pressure directed by said openings to opposite surfaces of the dia.-. phragm,

2. A microphone comprising a housing, a single diaphragrnmounted in said housing, a generating' element actuable by said diaphragm, said housing having a plurality of spaced sound pas,- sages therein leading to opposite faces of and substantially equi-spaced from said diaphragm,

said openingsbeing spaced at different distances from any sound source, at least two spaced pase sages leading to one face of said diaphragm and the mouthof each being spaced from the mouth of. a passage leading to the other face of said dia;

phragman equal distance, the mouths of all pas-- sages-being located in asubstantially common plane; said diaphragm being responsive to the seconddifferential of the sound pressure applied thereto through said passages with respect to the various distances between the source of sound and the mouths of said passages. 1 3. A microphone comprising a housing, a single diaphragm mounted in said housing, a generating elementfactuable by said diaphragm, said housing having a plurality of spaced similar sound passages therein leading to opposite faces of and substantially equally spaced from said diaphragm, at least two spaced passages leading to one face of said diaphragm whereby the diaphragm is sub! jected to the differences between sound pressure applied through each thereof and a pressure applied to the opposite face of said diaphragm, the mouths of said passages being located in a substantially common plane and the mouths of s id two passages leading to one side of. said dia- Phr gia bein oca d at o t i es he wail; o a sassase ead o h oth whereb diaphragm and having substantially the same acoustic impedance, at least two spaced passages leading to one face of said diaphragm whereby the diaphragm is subjected to the differences between sound pressure applied through each thereof and a pressure applied to the opposite face of said diaphragm, the mouths of all of said passages being spaced and located-with respect to the plane 'ofsaid diaphragm a small fraction of the spacing between different openings leading to opposite sidesof said diaphragm.

5 A microphone comprising a housing, a diaphragm mounted in said housing and dividing said housing into two compartments, and a gene crating element actuable by said diaphragm, said housing having three openings therein whose months he in a common plane substantially par:

vallel to said diaphragm and whose axes lie in another common plane substantially perpendicular to said first plane and are spaced apart uniformly, the two end openings communicating with one of said compartments and the intermediate opening communicating with the other compartment.

6. A microphone as defined in claim 5, wherein the mouths of said openings are arranged with one thereof displaced laterally from alignment with the other two a distance less than one-half the distance between adjacent openings.

.7. A microphone as defined in claim 5 wherein the mouths of said openings are arranged and displaced from a common plane by an amount less than one-half the distance between adjacent openings.

' 8. A microphone comprising a casing having a substantially flat wall, a diaphragm mounted in said housing substantially parallel to said wall and dividingsaid housing into two compartments, and a generating element actuable by said diaphragm, said housing having two similar sound passages communicating with one chamber whose mouths open at said wall and a third passage of substantially the same acoustic impedance as said first'passages communicating with the other chamber whose mouth opens at said wall between and equi-spaced from the mouths of said first passages and substantiall in line therewith,

' 9. A microphone comprising a houing having a substantially flat Wall, an annulus carried by said wall and projecting into said housing, a diaphragm mounted on and spanningthe inner end of said annulus intermediate the depth of said housing, an electrical generating element actuable by said diaphragm, said wall having an opening therein communicating with the space with-' in said annulus and having two openings therein communicating with the interior of said casing outwardly of said annulus, the mouth of said first opening being equispaced from and substantially aligned with the mouths of the other open} 9 ings and all openings having substantially the same acoustic impedance.

10. A microphone comprising a housing having a chamber and defined in part by a removable Wall portion having a substantially flat outer surface, a diaphragm carried by said wall and spanning and dividing said chamber and substantially parallel to said outer wall surface, a conductor carried by the portion of said housin opposite said wall portion, a retainer interposed between said diaphragm and conductor;and a body of granular conductive material confined by said retainer in contact with said conductor and an intermediate portion of said diaphragm, and at least three passages of substantially equal acoustic impedance open at said Wall in equispaced substantially aligned relation with alternate passages substantially equally spaced from said diaphragm and communicating with different portions of said chamber to impress sound against opposite faces of said diaphragm.

11. A microphone comprising a housing having a chamber and a peripheral wall, a diaphragm spanning said chamber, and an electrical generating element actuable by said diaphragm, said wall having a plurality of pairs of passages therein of similar acoustic impedance and whose mouths are substantially equally spaced from said diaphragm, one passage of each pair impressing sound upon one face of said diaphragm and the other passage impressing sound upon the other face of said diaphragm, the passages of each pair being displaced the same distance and the axes of the mouths of all of said passages lying substantially in a common plane.

12. A microphone as defined in claim 11, wherein the openings of each pair bear the same relation to each other peripherally and longitudinally and any displacement of the passages of a pair in a longitudinal direction is less than one-half the total distance between the openings of each pair.

ALPHA M. WIGGINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,546,749 Roberts July 21, 1925 2,301,744 Olson Nov. 10, 1942 FOREIGN PATENTS Number Country Date 336,387 Great Britain Oct. 16, 1930 

